The present emphasis in the development of information storage systems is the capability to store more and more information into a so-called "desk top" sized computer memory system. Those "desk top" sized memory systems which incorporate magnetically recorded hard disk media, such as that used in Winchester disk drive type memory systems, currently have the capacity to store upwards of 20 megabytes of magnetically recorded information. The problem with such memory systems is that by necessity the hard disk media is permanently mounted into the computer. Since the media is not easily removable, the use is limited to whatever portion of the hard disk is remaining for information storage at the time of use. Accordingly, magnetically recorded hard disk media information storage systems are not viewed as a potential solution to increasing information storage capacity.
So-called "floppy" disk memory systems wherein flexible disks, each having a diameter of either 5.25 inches or 3.50 inches, are used as the storage media provides easily removable storage media. However, the problem with such storage systems is that the present storage capacity of information magnetically recorded on a single floppy disk used in such a system has not yet reached a level equal to that of the hard disk, i.e., a single floppy disk media can only store approximately 1 to 2 megabytes of magnetically recorded information.
Systems for storing information which can be accessed through optical devices have received significant attention due to their potential capacity to store substantially more data, i.e., on the order of 400 to 800 megabytes of information, than that available in either magnetically recorded hard disk or floppy disk storage systems.
The improved capacity of optical memory is obtained at a higher cost for the media, as well as for the drive when compared to magnetic memory devices. New advances in barium-ferrite (BaFe) magnetic media allows bit densities to exceed optical bit densities. However, track densities of removable magnetic media are many times less than their optical counterparts.
A disk drive which uses an optical track sensing technique to determine the radial position of a magnetic head on a disk has been used to allow higher track densities. One such system is described in AN INTRODUCTION TO THE INSITE 325 FLOPTICAL.RTM. DISK DRIVE, Godwin, in a paper presented at the SPIE Optical Data Storage Topical Meeting (1989). This disk drive uses disks containing prewritten optical tracks with a 20 micron pitch. A light emitting diode illuminates the disk. The image of the disk's surface is transferred to a four-element photodetector by a lens and mirror. Such a system has the following drawbacks.
Because the INSITE head uses an L.E.D. as a diffused illumination source, light intensity at the disk, and subsequently on the detector, is very low. This creates a very low level tracking signal which must be greatly amplified electronically. Any system noise at this point also gets amplified and a signal with a high degree of undesired noise is obtained.
The INSITE head senses a very short segment of only two tracks. Since such a small area of the tracks is being detected,then flaws in the disk such as small debris, scratches, roughness of the edges of the optical tracks, or even reflectivity variations all contribute to noise.
The head of the INSITE optical system creates a magnified image (5.5 X) of the surface of the disk with a very short depth of focus (.+-.6 .mu.m). Because of this short depth of focus, the tracking signals degrade very quickly in the presence of shock, vibration, and thermal expansion within the head. INSITE includes a bi-metallic mechanism to compensate for thermal expansion and index of refraction changes within the optical system.